The present invention relates to a system for extracting samples from a stream flowing in a conduit, which is portable and operates on either an isokinetic or a constant volume sampling basis.
Constant volume sampling (CVS) has been recognized as the most accurate way of measuring pollutant emissions under conditions of varying flow rate, for example, exhaust emissions of a vehicle driven at varying speeds. In a CVS system, the exhaust stream containing the pollutants is diluted with clean air such that the combined flow rate of the mixture of exhaust gases and clean air is substantially constant. As a result, the pollutant concentration in the diluted mixture is proportional to the mass flow rate of pollutants in the exhaust stream. Moreover, particulate components are condensed and measured. However, such CVS sampling systems in existence are relatively large and expensive. As a practicality, these systems do not accurately measure exhaust streams from large engines, such as those used in locomotives and heavy trucks. In addition, such systems are not portable.
Portable emission sampling systems have also been developed. However, such systems are generally not CVS systems. Instead, prior portable emissions sampling systems rely on measurements of pollutant concentration in the undiluted exhaust and are combined or compared to a direct or indirect measurement of exhaust flow rate. Unfortunately, these portable pollutant concentration systems are inaccurate, since the measuring or estimating of exhaust flow rates varies from vehicle to vehicle. In addition, present pollutant concentration measurement systems are unable to determine particulate emissions accurately.
U.S. Pat. No. 5,090,253 describes a flow meter which is used to measure fluid from an exhaust conduit.
U.S. Pat. No. 5,333,511 shows a batch exhaust analyzer which is programmed to obtain samples at certain times.
U.S. Pat. Nos. 4,586,367, 4,633,706, 4,654,058, and 4,747,297 describe apparatuses for measuring particulate matter in exhaust streams, with or without the use of dilution tubes.
U.S. Pat. Nos. 5,058,440, 5,101,670, 5,218,857, and 5,410,907 show pollutant concentration sampling devices which use dilution tunnels. U.S. Pat. Nos. 5,184,501 and 5,337,595 employ pollutant concentration type samplers which also measure bulk stream flow rates.
A paper entitled "The Measurement of Gaseous and Particulate Emissions from Light-Duty and Heavy-Duty Motor Vehicles Under Road Driving Conditions" by Potter, C. J. et al describes an emission sampling system which operates on a CVS principle. This system employs a passive cap for the end of the vehicle exhaust pipe to divide the exhaust flow among a large number of identical parallel small pipes. One of the pipes leads to a dilution tunnel. To maintain proportional flow through all of the pipes, the system requires that the pressure drop across the cap be much greater than the pressure drop between the cap and the dilution tunnel. Changing the pressure in the exhaust system affects the generation of pollutant emissions. Inaccurate measurements ensue, especially in turbocharged engines. In addition, the system is unwieldy for mounting to a vehicle.
In measuring particulate emissions from exhaust streams, such as chimney outflows having particles larger than a few microns, it is important to maintain isokinetic sampling conditions. In other words, the velocity of the gas entering a sample probe must possess substantially the same velocity as the gas in the exhaust stream. If such condition is not achieved, the inertia of particles in the exhaust stream may result in a sample being enriched or depleted in particles, relative to the concentration of particles in the exhaust stream. U.S. E.P.A. Method 5--"Determination of Particulate Emissions from Stationary Sources" documents a standard procedure for measuring particulate emissions in stationary exhaust stacks, using isokinetic sampling. The method requires the use of a separate pitot tube and orifice meter. Manual adjustments and calculations achieve isokinetic sampling only under steady state conditions.
A system which is capable of automatically extracting a proportional or isokinetic sample of gas and/or particulate emissions from an exhaust stream under varying flow rates, which is portable, accurate, and does not interfere with the exhaust stream would be a notable advance in the field of emission measurements.